Students who stay up late to cram for a test or finish a project have lower comprehension and worse performance in the classroom as a result, research shows.

The old aphorism that "you snooze, you lose" doesn't apply to students who stay up late to cram for a test or finish a class project. New research shows that sacrificing sleep for school work is a bad trade.

Researchers from UCLA's Jane and Terry Semel Institute for Neuroscience and Human Behavior enlisted students from three Los Angeles high schools to help them figure out whether academic performance suffered the day after a late night of studying.

It turned out their hunch was correct: Lost sleep resulted in less comprehension during class and worse performance on tests, according to their report, published online Tuesday in the journal Child Development.

"Sacrificing sleep for studying seems to be counterproductive," said Andrew J. Fuligni, a developmental psychologist at UCLA and the study's senior author.

The researchers gave 535 teenagers checklists to keep track of their sleep and study time for three 14-day periods when they were in ninth, 10th and 12th grades.

The UCLA team found that regardless of how much time a high schooler normally spends on homework each day, a student who gives up sleep for extra study time will have trouble the next day understanding material in class and be more likely to struggle with an assignment or test — the opposite of the student's intent.

The researchers didn't quantify the increased risk for academic problems following a longer-than-usual study session, but they said the number of problems was "surprisingly greater."

The relationship held up no matter how academically ambitious the student was, as measured by the amount of time spent studying on a typical day, and it became stronger as students progressed through high school.

The results rang true to Kai Daniels, a college-bound senior at the Los Angeles Center for Enriched Studies, a Mid-City magnet school. On occasions when she's stayed up late to study, she's had more trouble absorbing material in class, she said.

"I'd have to re-teach myself at night," she said.

The finding "makes a lot of sense," said Mona el-Sheikh, a professor of human development and family studies at Auburn University whose research includes sleep. Several new studies are showing that the quantity and the quality of sleep are important for remembering new information and consolidating learning, she said.

Students who get too little sleep don't have enough time to process what they study, she added; even just one night of sleep deprivation can have a negative effect. Parents should do what they can to make sure their children have sufficient and consistent sleep, she said.

Fuligni said he could not disclose which schools took part in the research. The students varied in ethnic and economic backgrounds, as well as in their level of academic achievement. Their checklists revealed that study time did not change over the course of high school — the average was just over an hour per day — but sleep time decreased by an average of 41.4 minutes.

He said the research didn't delve into why things got worse over time. But as the parent of a teenager, he offered several theories: Perhaps it's because the work grows more challenging, or that the teenagers are biologically driven to stay up later but still have to rise early because of school start times. They also may have developed other interests, including jobs.

Most adolescents need just over nine hours of sleep a night, which 9% of high school students actually get, according to the National Sleep Foundation. Though a consistent study and sleep schedule are ideal, the demands that high school students face make that "infeasible," the researchers wrote.

Fuligni suggested that students do their best to compensate by distributing study time evenly over the week. When extra time is needed, they should consider cutting back on an activity other than sleep. Fuligni's previous research showed as many as four extra hours were available from time spent socializing, watching TV and helping the family.